1. Field of the Invention
The present application is related to a backup power supply, and more particularly to a backup power supply device which is suitable for being used in a handheld PC, and which has different selectable modes for providing propriate electricity to the handheld PC.
2. Description of Related Art
Generally, when the handheld PC is under a low battery situation, a standard mode or a premium mode is always adopted for managing the power system. In the standard mode, when the electricity of the main battery is exhausted or is removed, the data of the handheld PC will automatically be stored in the memory and the backup battery will provide power to the memory and the real time clock (RTC) for at least 30 minutes so as to maintain the data and also the time. Moreover, in the premium mode, the backup battery only provides power to the RTC so as to maintain the time information for about 168 hours.
Please refer to FIG. 1, which is a schematic view showing the conventional power supply device in the handheld PC. The power supply device 1 includes a main battery 14, a charging unit 19, a backup battery 18 and a power outputting unit 16. When the handheld PC is under a normal power supply situation, the main battery 14 provides voltage to a RTC 2 and a memory unit 3 and, at the same time, charges the backup battery 18 through the charging unit 19. Here, the memory unit 3 is usually a SDRAM (Synchronous Dynamic Random Access Memory).
However, the power supply device 1 as shown in FIG. 1 only can be used in the standard mode. When the electricity of the main battery is consumed or the main battery is removed, the handheld PC is changed to be supplied by the backup battery 18. At this time, the backup battery 18 outputs backup voltage to the power outputting unit 16, so that the power outputting unit 16 can provide voltage to the RTC 2 and the memory unit 3. Here, the power provided by the backup battery 18 is used for maintaining the integration of data, including data in editing, program under using and other set values. Besides, the RTC 2 can maintain the time information of the handheld PC.
Besides, since the backup battery 18 has to provide power to RTC 2 and memory unit 3 at the same time, the power only can maintain the system for about 30 to 90 minutes, and after the power is consumed, the data stored in the memory unit 3 will disappear.
Therefore, the conventional power supply device 1 can not provide the user the selection between standard mode and premium mode, so that only the standard mode can be provided for managing power, in which the provided power actually is insufficient in maintaining a long term operation of memory unit 3 and RTC 2.